Secondary batteries contain nonaqueous electrolyte solutions. The nonaqueous electrolyte solutions contain nonaqueous solvents such as cyclic carbonates such as ethylene carbonate and propylene carbonate; linear carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; linear esters such as methyl acetate and methyl propionate; cyclic ethers such as tetrahydrofuran, 2-methyl tetrahydrofuran, and tetrahydropyran; linear ethers such as dimethoxyethane and dimethoxymethane; and sulfur-containing organic solvents such as sulfolane and diethyl sulfone and also contain electrolytes, such as LiPF6, LiBF4, LiClO4, LiCF3SO3, LiAsF6, LiN(CF3SO2)2, and LiCF3(CF2)3SO3, dissolved in such nonaqueous solvents.
Patent Document 1 cited below discloses that difluorophosphates are useful as stabilizers for polychloroethylene. Patent Documents 2 and 3 cited below disclose that the difluorophosphates are useful as additives for nonaqueous electrolyte solutions for lithium secondary batteries. However, Patent Document 3 also discloses that a battery containing a mixture of lithium difluorophosphate and lithium monofluorophosphate has properties inferior to those of a battery containing lithium difluorophosphate. Therefore, it has not been clear that what kind of difluorophosphate is preferred and advantages of the difluorophosphates and conditions for using the difluorophosphates also have not been clear.
Non-patent Documents 1 and 2 disclose methods for producing the difluorophosphates by the reaction of P2O3F4 with metal salts or NH3. Non-patent Document 3 discloses that the difluorophosphates are produced by the reaction of difluorophosphoric acid with metal salts.    Patent Document 1: U.S. Pat. No. 2,846,412    Patent Document 2: Japanese Patent No. 3439085    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-31079    Non-patent Document 1: J. Fluorine Chem. (1988), 38 (3), 297    Non-patent Document 2: Inorganic Chemistry (1967), 6 (10), 1915    Non-patent Document 3: Inorganic Nuclear Chemistry Letters (1969), 5 (7), 581
P2O3F4, which is a source material used in the methods described in Non-patent Documents 1 and 2, is not readily available and is very expensive. The difluorophosphates produced by the methods need to be separated from by-products and then purified.
High-purity difluorophosphoric acid used in the method described in Non-patent Document 3 is not readily available. In this method, many steps need to be performed in the absence of moisture.